Electrolytic polishing of stainless steel



United States Patent i 3,213,008 ELECTROLYTIC POLISHING 0F STAINLESS STEEL Sadi C. Valentin, Milan, Ill., assiguor to Ametek, Inc., a corporation. of Delaware N0 Drawing. Filed June 14, 1961, Ser. No. 116,932 2 Claims. (Cl. 204-1405) The present invention relates to the electrolytic polishing of stainless steel, and to a method therefor.

The electrolytic polishing of metals including stainless steel is well known. Prior known methods for such polishing of stainless steel have proven somewhat unsatisfactory because the polished part often lacks the smoothness and brilliance demanded in the trade.

The manner in which a metallic surface is polished by the electrolytic method is essentially an electrolysis process in which the metal to be polished is made the anode in the circuit. If the metal or alloy is active the chief reaction will be an oxidation of the metal or metals to positively charged ions in solution. This in its simplest terms means that the surface of the metal anode dissolves into the solution due to the effect of the electric current and the amount of metal dissolved is directly proportional to the quantity of current that is used.

The tendency of a metal to oxidize to one of its saltlike ions is directly measured in volts and depends on the kind of metal, its concentration in the solution, and on the temperature. This tendency is called the electrode potential of the metal. 'For example, iron has a tendency to pass into solution in an electrolyte containing one gram atomic weight of iron (56 grams) in the ferrous state and at room temperature. This tendency when measured is found to be 0.440 volts. The equation is Fe=Fe+++2 electrons; E" (voltage) =0.44

Similarly:

Ni (nickel)=Ni+++2 electrons: E=0.250 Cr (chromium)=Cr++++3 electrons: E=0.71 Mn (manganese) =Mn+++2 electrons: E=0.250

From these measurements one finds that manganese has the highest tendency to pass into solution, chromium is next, then iron, and finally nickel, these being generally the metals found in stainless steel.

These voltages refer to the metals in their pure and isolated state. In stainless steel they are in the form of an alloy and consequently their electrode potentials may vary from these values considerably. Furthermore, surface strains are usually present, particularly in stainless steel, which are capable of producing further variations in potential measurements.

In general, however, one can assume that in an electro- -polishing method, the ease with which each particular metal is dissolved off the surface of the alloy is roughly proportional to its electrode potential. For example, if the voltage across an electrolytic bath is increased until dissolution of the anode just begins, manganese would be the first of the metals in stainless steel to pass into solution. An increase in voltage (and therefore an increase in current density) will allow chromium to also pass into solution. Thus a step-like increase in current can be made until all the metals in the-alloy will readily dissolve. If a further increase in current density takes place, a potential will eventually be reached at which point water is decomposedinto oxygen gas and the acid hydrogen ion. Whenoxygen is freely evolved around the anode, a film of the gas may form on the electrode surface, causing polarization and thus hindering the electropolishing ac- 3,213,008 Patented Oct. 19, 1965 tentials) can be overcome by vigorous stirring. An increase in temperature of the solution will also help since the fluidity increases with temperature.

Theoretically, one should expect the following to be true in a process of electropolishing metals:

(1) There should exist a minimum current density below which the metal surface of an alloy is not uniformly polished.

(2) There should exist a maximum current density above which harmful effects would appear which would not give a good polished surface.

(3) The range between the minimum and maximum current densities should be the range in which satisfactory polishing may be obtained, provided a satisfactory electrolyte is used and efficient stirring is employed. sufficient time must be given to allow enough of the metal surface to dissolve away to give a smooth surface.

There is, however, one other factor to'consider: the composition of the electrolytic bath. It is well known that in a plating process, chromium can be plated out at the cathode from a concentrated solution of chromic acid containing sulfuric acid or chromic sulfate. This process uses up hydrogen ions (acid) and a film of the basic chromous chrom-ate, Cr(OH)CrO appears to be formed, first on the cathode. The sulfate ion, when present, acts in some way to break up this film, and the chromium in the film is then readily reduced to the metallic state.

In the electrolytic method of polishing stainless steel, one is concerned with the reverse procedure, namely, oxidizing the metallic chromium to soluble chromium ions. It may be that a film is first produced on the anode, which must be broken down before good results are obtained.

The principal object of this invention is to provide an electrolytic polishing method that will produce a polished article having a greater degree of smoothness than attaina-ble with prior known processes.

Another object of this invention is to provide such a polishing method that will produce a polished surface of greater brilliance than that produced by known processes.

Still another object of this invention is to produce an electrlytic bath that will polish stainless steel in a manner to produce polished articles of greater smoothness and brilliance than can be achieved with prior known processes.

In one aspect of the invention, an electrolytic bath may contain sulfuric acid, phosphoric acids and water in substantial amounts.

In another aspect of the invention predetermined amounts of chromium, ferrous, nickel and aluminum sulfate may be added.

In another aspect of the invention, silicic acid may be substituted for the aluminum sulfate.

In still another aspect of the invention, aluminum sulfate and silicic acid may be employed.

In still another aspect of the invention, magnesium sulfate may be used instead of the aluminum sulfate and silicic acid.

The procedure for making a ten-gallon bath of the electrolyte may comprise pouring 3.48 gallons of water into a vat and adding about one quart of sulfuric acid (66 B.) while continuously stirring with a rod or paddle made of porcelain or glass. To this solution is slowly added while stirring, 1.355 pounds of Cr (SO 1.177 pounds of FeSO 0.269 pounds of NiSO, and 0.158 pounds of Al (SO The acid is added to the water to prevent hydrolysis of the salts in the solution. When these salts are completely dissolved, the remaining sulfuric acid (3.25 gallons), and the remaining phosphoric acid (3.61 gallons) areadded slowly while stirring so as to prevent spattering which might occur if they are added too rapidly.

This solution preferably is stirred several minutes at one hour intervals for a half a day or so and then allowed to. stand overnight or longer in order to ensure uniform dispersion of the components. Since the solution is hygroscopic, the bath should be covered when not being stirred to prevent its absorbing moisture from the air.

If the salts used contain water of crystallization, the quantity used should be calculated to allow for this increase in weight. The water present in the salt should be subtracted from the amount of water added in the formulation of the solution.

The solution is now ready for use.

The above, other objects and novel features of the invention will become apparent from the following specification which is merely exemplary.

As examples of electrolytic baths and the conditions under which they produce good polishes on stainless steel, the following are cited:

Bath #1 Percent Phosphoric acid. 75%

Sulfuric acid, 66 Be 49. 58 38. 40 Water 28. 58 22. 1O Chromium sulfate 1.

Ferrous sulfate 1. 31 2 70 Nickel sulfate 45 Aluminum sulfate 21 Bath #IA Phosphoric acid, H3PO4 27. 11 Sulfuric acid, H 804 36. Water, lEl O 34. 48

Ferrous sulfate, FeSO4 Nickel sulfate, NiSOA Aluminum sulfate, M 604);

Bath #2 Phosphoric 'acid, 75%---. Sulfuric acid, 66 Be Samples of cold rolled sheet stainless steel, type 302, were made in two-inch squares giving approximately eight square inches of surface to the samples. Two small holes were punched in each sample through which bolts fastened the sample to copper lead wires.

After the sample is tightly bolted to the lead wires, it is degreased by washing in trichloroethylene vapors, rinsed with hot water, placed for one minute in an electric cleaning bath, scrubbed off with pumice stone, and again rinsed with water. The sample is now ready to be placed in the polishing tank. (The sample is not touched with the hands after the degreasing step.)

4 The conditions selected were:

Time 10 minutes. Air stirring Constant rate of bubbling through solution. Temperature 185 to 190 F.

Under these conditions the current density was changed over twenty-five sample runs to determine the optimum current density which gave consistently good polishing. It was found that this optimum was reached at 58 amperes for a lO-minute run. The total surface drawing this current (sample plus lead wires) is approximately 10 square inches. This is equivalent to a curent density of 5.8 amperes per square inch. The cathodes used inthe early runs were made of stainless steel, type 316', but these were changed to type 317 which gave better results. it is im portant to keep the cathodes clean.

After determining the optimum current density for the 10-minute run, this Value was held constant along with stirring, temperature, sample (stainless steel, type 317), and the range of time of polishing determined over which consistently good results were obtained on the fiat samples. This required twenty-eight more runs. Samples polished over a range of time from 6 to 12 minutes gave good results. The best results were obtained at 9 to 10 minutes.

A sample of stainless steel, type 309', was given a 10- minute polish under the above conditions and a good polish was obtained.

A sample of hot rolled stainless steel, type 347, was first pickled to remove scale and then polished for 10 minutes. A good polish resulted but the pitted surface of the sample remained.

To check the bath using plant run parts, two bumper ends were polished. The exact surface area was not determined, but an excellent polish was obtained using amperes for 10 minutes at F.

The surface of the samples polished in Bath #1 ap-= peared both to the eye and on microscopic examination to be the most uniform of all samples polished, but showed a slight grayish cast.

The surface of the samples polished in Bath #2 showed more brilliance to the eye than that of the samples polished in Bath #1.

The samples polished in Bath #3 showed a much smoother surface than was obtained in Bath #2, but possessed more brilliance than any of the other samples with the exception of the samples polished in Bath #2.

The samples polished in Bath #4 had a grayish cast similar to that on the samples run in Bath #1.

Larger samples were successfully polished in each of the above solutions. The dimensions of these samples were 4 x 4, 6 x 6, 8 x 8, and 8 x 10 inch squares. These were run in order to see if larger flat surface areas can be given a uniform polish using the solutions of this invention.

The surface of the polished samples were examined under the low-power lens of a microscope. The unpolished sample shows the lines parallel with the direction of rolling. The shearing appears to have folded the surface of the metal over, giving a scale-like effect.

The samples polished for two minutes lost the scale-like surfaces and had irregular surfaces with many pits of volcanic appearance. Some of these pits or craters seemed to be covered, thus producing blisters on the surface.

All of the polished samples have a similar type of surface, the crater-like pits smoothing out somewhat as the time of polishing increases. The surfaces which seem to be most free of these craters are the samples polished in Bath #1 (the one containing aluminum). To the eye these appear to have the smoothest surface. Samples polished in Bath #2 were next in smoothness; however, all indications point to the presence of aluminum in the: bath as a contributing factor in producing a smooth sur-- face.

It is difficult to determine the most br llian S but the presence of silicon in the bath does add brilliance to the polished metal.

Although the various features of the new and improved process for electrolytically polishing stainless steel have been described in detail to fully disclose several embodiments of the invention, it will be evident that changes may be made in such details and certain features may be used without others without departing from the principles of the invention.

What is claimed is:

1. The method of electrolytically polishing stainless steel which comprises making such stainless steel the anode in an electrolytic bath at a current density, time and temperature so related as to dissolve away the surface to give the desired smooth surface, said bath consisting essentially of phosphoric acid of about 37% by Weight; sulfuric acid of about 38% by weight; Water of about 22% by weight; chromium sulfate in the range of about 1.1% by weight; ferrous sulfate in the range of about 1% by weight; nickel sulfate in the range of about 0.4% by weight; and aluminum sulfate in the range of about 0.2%

by weight.

2. The method of electrolytically polishing stainless steel which comprises making such stainless steel the anode in an electrolytic bath at a current density, time and temperature so related as to dissolve away the surface to give the desired smooth surface, said bath consisting essentially of phosphoric acid between the limits of about 27-37% by weight; sulfuric acid between the limits of about 36-38% by weight; water between the limits of about 34-22% by weight; chromium sulfate in the range of about 1.1% by weight; ferrous sulfate in the range of about 1.1% by weight; nickel sulfate in the range of about 0.4% by weight; and silicic acid in the range of about 0.1% by weight.

References Cited by the Examiner UNITED STATES PATENTS 2,334,699 11/43 Faust 204140.5 2,357,219 8/44 Mott 204l40.5 2,429,676 10/ 47 Faust 204140.5 2,440,715 5/48 Faust 204140.5 2,776,255 1/57 Hammond 204-1405 2,997,429 8/61 Rohrer et a1 204-140.5

FOREIGN PATENTS 556,797 10/43 Great Britain.

JOHN H. MACK, Primary Examiner.

WINSTON A. DOUGLAS, Examiner. 

1. THE METHOD OF ELECTROLYTICALLY POLISHING STAINLESS STEEL WHICH COMPRISES MAKING SUCH STAINLESS STEEL THE ANODE IN AN ELECTROLYTIC BATH AT A CURRENT DENSITY, TIME AND TEMPERATURE SO RELATED AS TO DISSOLVE AWAY THE SURFACE TO GIVE THE DESIRED SMOOTH SURFACE, SAID BATH CONSISTING ESSENTIALLY OF PHOSPHORIC ACID OF ABOUT 37% BY WEIGHT, SULFURIC ACID OF ABOUT 38% BY WEIGHT; WATER OF ABOUT 22% BY WEIGHT; CHROMIUM SULFATE IN THE RANGE OF ABOUT 1.1% B Y WEIGHT; FERROUS SULFATE IN THE RANGE OF ABOUT 1% BY WEIGHT; NICKEL SULFATE IN THE RANGE OF ABOUT 0.4% BY WEIGHT; AND ALUMINUM SULFATE IN THE RANGE OF ABOUT 0.2% BY WEIGHT. 